Turbocharger systems for supercharging diesel-type internal combustion engines, preferably for heavy-duty vehicles, usually comprises a single-stage compressor driven by a single-stage turbine, both of the radial type. Turbocharger systems with two-stage supercharge are also found, sometimes also including intermediate cooling, but such installations are usually very space-consuming. Technical solutions of this kind are therefore difficult to apply to motor vehicles in which space is severely limited.
Traditional turbochargers used in two-stage supercharging systems have to be placed with their axes laterally offset, since the flow leaving the high-pressure turbine is axial, while the inlet to the low-pressure turbine is radial. Hence, there has to be a 90° bend in the gas duct. This bend takes up a lot of space and causes large pressure losses, which have an adverse effect upon the overall efficiency.
Superchargers suitable for a diesel engine of 6 to 20 liters cubic capacity normally have an efficiency, under stationary conditions, of between 50% and 60% (ηcompressor*ηmechanical*ηturbine). In contemporary diesel engines the efficiency benefit is lower than for future engines which will require higher charging pressure. Examples of systems which raise the supercharging requirement are exhaust gas recirculation for lower nitrogen oxide emissions or systems offering variable control of inlet valves.
Turbocharger systems with higher efficiency than 60%, under stationary conditions, offer a greater prospect of meeting future demands for environmentally friendly and fuel-efficient engines. Until recently, environmental demands upon diesel engines have usually led to worse efficiency meaning that the energy resource of the fuel is not best utilized.